The present invention relates to parachute canopies and more particularly to ribbon parachute seam joints for joining a ribbon seam and a radial seam of a ribbon parachute canopy together Further, the present invention addresses the problem of seam strengthening.
A ribbon parachute is a very strong and very stable type of parachute. It functions well at supersonic speeds as well as transitional and subsonic speeds. The parachute consists of a canopy, suspension lines, and risers. The canopy is the cloth or other similar portion of the parachute. Seams are generally considered part of the canopy. Specifically, a ribbon parachute canopy has ribbons, seams, and radial tape. The purpose of the ribbons is to generate drag forces on the parachute. The ribbons also function to transfer these drag forces to the radial tape. The radial tape provides the framework for the canopy and is a primary load transferring member of the canopy. The radial tape transfers radial and circumferential canopy forces to the parachute suspension lines. The seam joints of the present invention are the place where the ribbons and radial tape are joined so as to accomplish the transfer of radial and circumferential forces from the ribbons to the radial tape. Cloth parachutes, unlike ribbon parachutes, have a continuous canopy and therefore do not have seam joints per se but rather have traditional seams which accomplish the transfer of forces.
Parachute seams and seam joints are typically the weakest part of the parachute. Present seams and seam joints maintain a strength of 75% to 90% of the minimum rated strength of the materials joined This translates to a seam efficiency of 0.75 to 0.90 respectively. Seam efficiency is often dependent on the properties and volume of material used in the fabrication of the seam. Typically, the greater volume used in the fabrication process of the seam, the greater the seam efficiency. Excess volume and hence weight, however, present additional problems to those involved in parachute design. The excess weight and volume of material increases the overall weight and volume of the parachute usually resulting in a decrease in available payload space which is of critical importance.
If the seam and seam joints possess a strength less than that of the materials being joined, this may result in seam yielding under normal stress loads and occasionally catastrophic seam failure under high load exposure. There is a continuing need to improve the strength of the seams and seam joints relative to the materials being joined and to improve the shock absorbing capacity of the seams. This improvement in seam strength must be accomplished while using a relatively small volume of material in fabricating the seam.
Prior art in this field involves numerous configurations of seams involving a variety of stitching patterns. (See FED-STD. No. 751). FED-STD. No. 751 is incorporated by reference herein In addition, the positioning of existing tape material to add reinforcement to the seam is a generally accepted design practice. In fact, ribbon parachute radial seams commonly consist of two strips of two inch wide radial tape attached to the ribbons with eight rows of straight stitching along the length of the strips of tape.
The present invention, however, provides unique configurations of a ribbon parachute seam and seam joint which combines a ribbon seam using a zigzag stitch pattern and narrow strips of radial tape sewn together with multiple rows of a straight stitch pattern. The ribbon seam attaches two parallel and narrowly overlapping ribbons within the parachute. The narrow strips of radial tape enclose the ribbon seam which results in a high strength and low volume radial seam and seam joint. This new configuration of a ribbon parachute seam has distinct advantages in terms of strength and shock absorbing capacity. Specifically, this new parachute seam has a seam strength equal to or greater than the minimum rated strength of the adjoining materials and employs a smaller weight and volume of material than conventional ribbon parachute radial seams.